Device for injecting an additive into the exhaust system of motor vehicle

ABSTRACT

Device for injecting a liquid additive including an additive tank connected by an additive pipe to an injector positioned in the exhaust system of the vehicle concerned, where there is provided, on the additive pipe, a positive-displacement lift-and-force pump including a cylinder and a piston which delimits within the cylinder a pressurizing chamber, and where an electric actuator is coupled to the piston to cause it to retreat, drawing additive into the chamber from the tank and simultaneously compressing a spring that acts on the piston, such that by virtue of the spring, the forward movement of the piston, delivering additive to the injector, takes place under controlled pressure.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a device for injecting an additive in the liquid state, such as urea, into the exhaust system of a motor vehicle with a combustion engine, particularly an engine that uses diesel fuel by way of fuel.

BRIEF SUMMARY OF RELATED ART

In motor vehicles that use diesel fuel by way of fuel, it is known practice, in an attempt to combat pollution, to add urea to the exhaust gases, particularly with a view to the reduction of nitrogen oxides.

More specifically, the urea is stored in a tank, in a dilute form diluted in water, and the additive thus formed is injected into the exhaust line of the vehicle, at an appropriate point in this line, that is to say upstream of the catalytic converter in which the reduction of the nitrogen oxides takes place.

It is therefore appropriate to pump the additive from the tank in which it is stored, and to do so in a suitably metered way, and to inject this pumped additive into the exhaust line, at sufficient pressure. The “logical” solution here is to add a metering pump, situated on the additive tank or between this tank and an additive injector positioned on the exhaust line (see, for example, patent documents FR 2 879 239 and WO 00/21881).

Such a solution entails the use of a relatively powerful pump, needed to pressurize a relatively large volume, for example 15 litres, corresponding to the capacity of the additive tank, whereas the flow rate of additive to be delivered remains very low, and needs to be injected only periodically.

Another solution, allowing the pump to be omitted, is to store the additive in a flexible pouch associated with a device that pressurizes the pouch, it being possible for this device to comprise a spring kept under compression and pressed against the said pouch—see document FR 2 870 172 or its equivalent WO 2005/113279, particularly FIG. 2.

However, a system such as this in itself is unable to control either the flow rate or the pressure of the additive and, in particular, does not adapt the pressure of the injected additive to suit the instantaneous requirements. As far as the amount of additive injected is concerned, or put differently, as far as the metering function is concerned, it is necessary to add at the injector an electrically operated valve controlled by a computer.

In practice, the devices for pressurizing and metering additives currently used in association with exhaust systems comprise a tank containing the additive, a (non-metering) pump for pressurizing this additive, and a metering means consisting of an electromagnetic injector which, for each electrical pulse received, delivers a calculated amount of additive.

For metering, the electromagnetic pulse may be of variable frequency and duration. In such a device, the additive pressure supplied by the pump needs to be regulated in order to ensure the accuracy of the volume of additive injected.

The pump, which operates continuously, has therefore to be specified on the basis of its largest flow rate and, when the injector is delivering very little additive, that is to say when the operating point of the combustion engine is at low pollution, the excess additive delivery has to be recirculated, which in itself represents a waste of energy.

In addition, current embodiments of such devices are such that, to ensure that they will operate when the vehicle is starting in extremely cold weather, typically at a temperature of below −11° C., which corresponds to the freezing point of the water-urea mixture, it is necessary to provide a sequence of emptying the additive circuit when the vehicle is stationary and also to locally provide heating means in the additive tank and in the lines.

The current embodiments do not provide an easy solution to the problem of controlling the movements of the additive from the tank to the injector and, during the purge phase, in the opposite direction, from the injector to the tank, irrespective of whether or not the vehicle combustion engine is running. Furthermore, the volume of additive to be drawn back up, which corresponds to the volume of the line leading as far as the injector, inevitably varies according to the vehicle, and a universal device would therefore be desirable.

One solution that has already been envisaged (French Patent Application No. 08.05185 filed on 22 Sep. 2008 in the name of the Applicant) provides, on the additive circuit, a member for pressurizing the additive (the member advantageously consisting of a piston mounted to slide in a cylinder) which is connected by a pipe that taps pressure from the vehicle fuel supply pipe. A pumping device such as this is therefore dependent on the fuel pressure in a line, which pressure is itself associated with the running of the combustion engine, and which disappears as soon as this engine is switched off, which means that a pressure accumulator has to be envisaged.

BRIEF SUMMARY OF THE INVENTION

It is an aim of the present invention to eliminate the various disadvantages set out hereinabove by providing a device for injecting an additive using pumping means suited to the operating conditions and requirements without being dependent on a fuel or any other pressure, these means also being suited to the purge function while at the same time constituting a relatively simple and economical solution to the problem set of pressurizing and metering the additive.

To this end, a subject of the invention is a device for injecting an additive in the liquid state, such as a mixture of water and of urea, into the exhaust system of a motor vehicle with a combustion engine, the additive injection device comprising an additive tank connected by an additive pipe to at least one injector positioned in the exhaust system and a means of pumping the additive being provided on the additive pipe between the additive tank and the injector, these means being capable cyclically of dispensing metered quantities of pressurized additive, the device being essentially characterized in that the pumping means includes a positive-displacement lift-and-force pump comprising a cylinder, a piston mounted such that it can move translationally in the cylinder and delimiting within this cylinder a pressurizing chamber, a motor or electromechanical actuator coupled to the piston so as to move it translationally, and spring means acting on the piston in the direction of forcing the additive from the cylinder in such a way that actuation of the motor or actuator in one direction causes the piston to retreat, drawing additive into the pressurizing chamber from the additive tank and simultaneously compressing the spring means, whereas the forward movement of the piston delivering the additive to the injector takes place under a controlled pressure, thanks to the said spring means.

Thus, the solution proposed by the invention uses motorized pumping means. Advantageously, the motor of the positive-displacement pump here is a DC electric motor which has the advantage that it can run in both directions of rotation, without difficulty, and that it has a high torque on start-up, this allowing the spring to be compressed easily, and also allowing this spring to be released with ease.

An electric motor such as this is advantageously connected via step-down gearing to a rotary endless screw mounted along the axis of the cylinder of the pump and collaborating with the piston shaped as a nut, or with a nut connected to the piston, the piston and/or the nut being prevented from rotating. A sealing membrane of the “rolling” type may be mounted between the piston and the cylinder of the pump in order fluid tightly to delimit the pressurizing chamber while at the same time being able to cope with the variations in volume of this chamber.

According to an important aspect of the invention, the electromechanical drive means are combined with a spring which, in practice, is a helical compression spring, mounted inside the cylinder of the pump and acting on the piston on the opposite side to the pressurizing chamber. The movement of the piston in the direction for drawing in additive is performed with the aid of the motor or actuator, the power of which needs to be sufficient to compress the spring and at the same time ensure that the pressurizing chamber of the pump is filled, and to do so in a short time, that is to say in a few seconds, for example in about five seconds. The movement of the same piston in the opposite direction, to deliver the additive into the pipe leading to the injector, is performed under controlled pressure thanks to the spring, the latter being rated in such a way as to obtain a small variation in force over the entire stroke of the piston so that the instantaneous flow rate of the injector varies little over this stroke of the piston. The spring in some way acts as a regulator of the pressure and flow rate of the additive during injection.

In a preferred embodiment of the additive injection device according to the invention, there is provided, on the delivery side of the positive-displacement pump, a three-way electrically operated valve, control of which allows the additive to be directed selectively to the injector or, conversely, to the tank in order to empty the additive pipe. The electrically operated valve “manages” the function of drawing up the additive and filling the pressurizing chamber, of delivering the additive, and also of emptying the lines. The device thus has an additional purge function, performed simply, that is to say returning the additive to the tank, thus preventing the lines from becoming blocked with frozen water-urea mixture. For this function of emptying the additive pipe, there is also provided, in combination with the aforementioned electrically operated valve, a venting check valve positioned on this pipe near to the injector.

The cylinder of the positive-displacement pump may comprise heating means, in particular means that work by circulating a hot fluid inside the wall of this cylinder, in line with the pressurizing chamber; these means locally heat the additive, at the pressurizing chamber delimited by the cylinder and the piston of the pump. Advantageously, the aforementioned three-way electrically operated valve is incorporated into the pump or attached to the cylinder of this pump and can thus itself also “benefit” from the aforementioned heating means which defrost the additive.

There may be interposed on the connection between the additive tank and the positive-displacement pump an auxiliary reserve of additive in the form of an annular chamber surrounding the cylinder of the pump in the region provided with the heating means, such that this auxiliary reserve also benefits from the defrosting effect.

Thanks to these arrangements, rapid action of the additive injection device is achieved, even in extreme cold, with priority heating of the water-urea mixture contained in the pump and its closest proximity, it being understood that the heating means call upon a fluid, present in the vehicle, the rise in temperature of which is itself rapid, for example the diesel oil in the fuel return line, or a combustion engine coolant itself heated by exchange of heat. The auxiliary reserve of additive makes it possible to make best use of the heat restored by the heat exchanger formed by the heating means through which a hot fluid thus runs. For preference, this auxiliary reserve of additive has a volume substantially equivalent to the amount of additive injected in an operating cycle of the pump, more particularly a cycle of filling the line that connects the pump to the injector.

BRIEF DESCRIPTION OF THE DRAWINGS

In any event, the invention will be better understood with the aid of the description which follows, with reference to the attached schematic diagram which, by way of example, depicts a number of embodiments of this device for injecting an additive into the exhaust system of a motor vehicle:

FIG. 1 is a block diagram of a device for injecting additive according to the present invention, in one position of the operating cycle;

FIG. 2 is a diagram similar to FIG. 1, but illustrating another position in the operating cycle;

FIGS. 3 and 4 are diagrams similar to the foregoing ones and illustrating the operation of the device during purging;

FIG. 5 is a detailed view in longitudinal section of the positive-displacement pump of this device, in one particular embodiment;

FIG. 6 is a sectioned view similar to FIG. 5, but showing another position of the positive-displacement pump;

FIG. 7 is a sectioned view similar to the foregoing ones, showing another embodiment of the positive-displacement pump.

DETAILED DESCRIPTION OF THE INVENTION

Reference is made first of all to FIGS. 1 to 4 which very schematically show a portion of the exhaust line 2 of a motor vehicle with a combustion engine, particularly a “diesel” engine, into which line an additive with a pollution-reducing function, particularly a mixture of water and of urea, is to be injected.

The additive is stored in an additive tank 3 which is connected, by an additive pipe 4, to an additive injector 5 positioned on the exhaust line 2. Interposed on the additive pipe 4 is a positive-displacement lift-and-force pump 6 designed to draw up the additive stored in the tank 3 and pressurize this additive so that it can be sprayed using the injector 5 into the exhaust line 2.

The positive-displacement pump 6 comprises a pump cylinder 7, in which there is slidably mounted a pump piston 8 which divides the cylinder into two chambers. An electromechanical actuator 9 is coupled to the piston 8 in such a way as to cause it to move translationally, in one direction or the other, inside the cylinder 7.

In one of the chambers delimited by the piston 8 inside the cylinder 7 there is mounted a helical compression spring 10 which bears against one face of the piston 8 so as to push it back to the right (with reference to the drawing). The other chamber 11 delimited by the piston 8 and the cylinder 7 constitutes a chamber for pressurizing the additive. The pressurizing chamber 11 is connected to the additive pipe 4 via a three-way electrically operated valve 12.

Two end-of-travel sensors 13 and 14 are provided on the pump cylinder 7 to detect the arrival of the piston 8 in one or other of its end-of-travel positions.

Finally, a venting check valve 15 is placed on the additive pipe 4, close to the additive injector 5.

FIGS. 1 and 2 illustrate the basic operation of the device, that is to say the cycle that allows additive to be injected into the exhaust line 2. More particularly, FIG. 1 shows the intake phase during which the three-way electrically operated valve 12 is operated in such a way as to place the additive tank 3 in communication with the pump 6, more particularly with the pressurizing chamber 11 of this pump. The electromechanical actuator 9 is then activated to move the piston 8 to the left, the piston 8 thus describing an intake stroke, progressively compressing the spring 10. At the end of its stroke, the pressurizing chamber 11 is filled with additive while the spring 10 is fully compressed. The electromechanical actuator 9 is then switched off.

Next, the three-way electrically operated valve 12 is switched, so as to place the pump 6, more particularly the pressurizing chamber 11, in communication with that portion of the additive pipe 4 that leads to the injector 5—see FIG. 2. The electromechanical actuator 9 is then switched back on, in the opposite direction, so as to release the piston 8 which, now subject to the action of the spring 10, describes a delivery stroke (to the right), thus pressurizing the additive contained in the chamber 11, the pressurized additive being made available in the portion of the additive pipe 4 that leads to the injector 5. The electromechanical actuator 9 is stopped either after a preset operating time or after it has effected a preset number of revolutions, this actuator 9 being particularly an electric motor with two directions of rotation (as specified hereinafter).

Arrival of the pump piston 8 at the end of its delivery stroke, as detected by the sensor 14, triggers the start of a new cycle identical to the preceding one, with a further switching of the electrically operated valve 12 and a further intake of additive into the tank 3, and so on.

On each operating cycle, the device prepares a constant amount of additive, which corresponds to the “swept volume” of the positive-displacement pump 6, in the knowledge that the stroke of the pump piston 8 is constant here. Furthermore, the control system controlling the additive injector 5 makes it possible to determine, by summing the unit quantities injected, the total amount of additive taken from the tank 3 and therefore also, through a simple calculation, the instantaneous position of the piston 8 with respect to its total stroke. The electronic computer (not depicted) which controls the device can then make a correction to the value of the additive pressure, associated with the variation in force of the spring 10 over the stroke of the piston 8.

The purge cycle designed to prevent the risk of blockage in times of extreme cold and, particularly, at temperatures of below −11° C., which corresponds to the temperature at which the water-urea mixture used solidifies, will now be described with reference to the next FIGS. 3 and 4. For this cycle the water-urea mixture is returned to the tank 3, using the same positive-displacement pump 6.

As the vehicle is being switched off, and depending on the detected outside temperature, the three-way electrically operated valve 12 is operated in such a way as first of all to place the chamber 11 of the pump 6 in communication with the tank 3. Thus, the piston 8 already drives towards the tank 3 the quantity of additive that was in the pump 6 at the instant the vehicle was switched off.

Next, when the piston 8 has reached the end of its stroke, the electrically operated valve 12 is switched, thus placing the chamber 11 of the pump 6 in communication with that section of the pipe 4 that leads to the injector 5—see FIG. 3. The actuator 9 is activated in such a way as to move the piston 8 to the left, so that the additive contained in the relevant section of the pipe 4 finds itself drawn into the pump 6, in the knowledge that the check valve 15 then allows air to enter the pipe 4, to replace the removed additive.

The actuator 9 is switched off when the piston 8 has reached the end of its retreat stroke as detected by the sensor 13. The electrically operated valve 12 is then switched and the actuator 9 is switched back on in the opposite direction so as to allow the piston 8 to move to the right, with the aid of the spring 10. The additive previously drawn back in by the pump 6 is then transferred to the tank 3. Emptying is thus performed, by delivering the additive from the pipe 4 to the tank 3.

FIGS. 5 and 6 depict in detail part of the additive injection device, particularly the positive-displacement pump 6 and its electromechanical actuator 9, in one particular embodiment, the elements that correspond to those already described being denoted by the same numerical references.

The actuator 9 here comprises a DC electric motor 16 with two directions of rotation, attached to the pump cylinder 7. The output shaft 17 of the electric motor is connected, via a step-down gearing 18, to an endless screw 19 mounted such that it can rotate about the axis A of the pump cylinder 7. The endless screw 19 collaborates with a nut 20 secured to the pump piston 8, the latter being prevented from rotating relative to the pump cylinder 7. Thus, the rotating of the electric motor 16, in one direction or the other, causes, through the gearing 18 and the screw-nut mechanism 19, 20, the axial translational movement of the piston 8, to the left or to the right, depending on whether the endless screw 19 is being screwed into or unscrewed from the nut 20, so as to deliver or take in additive.

In this embodiment, a flexible sealing membrane 21 of the “rolling” type is mounted between the pump piston 8 and the pump cylinder 7 in order fluid tightly to delimit, at least on one side, the pressurizing chamber 11.

In the region of this pressurizing chamber 11, the wall of the pump cylinder 7 is provided with heating means, in the form of a circuit 22 through which a sufficiently hot fluid that already exists in the vehicle concerned runs, an inlet for this fluid being provided at 23. The fluid in question is, for example, fuel such as diesel fuel returning to the fuel tank of the vehicle concerned. The circuit 22 creates a heat exchanger which allows firstly the additive contained in the pump 6 to be heated in order to obtain a rapid re-entry into service of the device for injecting this additive.

Around the region of the pressurizing chamber 11 there is also provided here an annular chamber 24 which constitutes an auxiliary reserve of additive, receiving at its inlet 25 the additive conveyed from the tank 3 by the pipe 4. This annular chamber 24, and therefore the auxiliary reserve of additive it contains, may benefit from the heating effect resulting from the circulation of a fluid through the circuit 22, because it surrounds this circuit 22.

At the outlet 26 from the annular chamber 24, the additive, possibly heated, is directed towards the three-way electrically operated valve which in this instance is separate from the positive-displacement pump 6.

In an alternative form illustrated by FIG. 7, the three-way electrically operated valve 12 is, on the other hand, incorporated into the positive-displacement pump 6 and, more particularly, positioned at the end of the pump cylinder 7 which is the end adjacent to the pressurizing chamber 11. Thus, the electrically operated valve 12 and the additive it contains may also benefit from the heating effect obtained by the circulation of a fluid through the circuit 22.

The swept volume of the positive-displacement pump 6 will be defined, for each practical application, on the basis of the desired autonomy for the device (expressed in terms of number of kilometres covered by the vehicle) and of the volume of the lines that are to be emptied; for example 30 cm³ corresponding to 30 km covered. The spring 10 of the positive-displacement pump 6 is rated to have a small variation in force over the entire stroke of the piston 8, so that the instantaneous flow rate of the injector 5 varies only little. This flow rate is dependent on the square root of the pressure, which is itself set, for example to a value ranging between 3 and 6 bar, by the spring 10.

Finally, it will be noted that, in the operation of the additive injection device described hereinabove, when the vehicle concerned is stationary, the pump 6 can be immobilized either in a position in which the pressurizing chamber 11 is full of additive, with the piston 8 retreated, or in a position in which the pressurizing chamber is empty, that is to say in which the piston 8 is advanced as far as an end-of-travel position (to the right). The choice of the position of immobilization is here dependent on the strategy adopted for heating and defrosting the additive.

As goes without saying and as is evident from the foregoing, the invention is not restricted only to those embodiments of this additive injection device which have been described hereinabove by way of example; on the contrary, it encompasses all alternative embodiments and applications thereof that follow the same principle.

In particular, the following would not constitute departures from the scope of the invention:

-   -   if, for driving the pump, the DC electric motor were to be         replaced by a motor of any other type, such as an electric         “stepping” motor, or any other electromechanical actuator;     -   if the single three-way electrically operated valve were to be         replaced by any equivalent means, such as two controlled check         valves respectively controlling the inlet and outlet of the         pump;     -   if the detail of the operating sequences were modified;     -   if the additive were heated by circulating any available fluid,         for example engine coolant, or alternatively if use were made of         electric heating means, particularly of the resistive type;     -   if the detailed shapes of the components of the device, for         example those of the pump piston, were modified, and if sealing         around this piston were achieved by any appropriate means, such         as simple annular seals;     -   if the additive injection pressure were adapted to suit each         particular application;     -   if the device were used with any type of additive tank, it being         possible for this tank to be rigid or, on the other hand,         created as a flexible pouch;     -   if the same injection device were to be used for the injection         of any kind of liquid additive, for example with cerine, in         conjunction with any type of combustion engine and fuel, on any         kind of vehicle.

The present invention relates to a device for injecting an additive in the liquid state, such as urea, into the exhaust system of a motor vehicle with a combustion engine, particularly an engine that uses diesel fuel by way of fuel.

In motor vehicles that use diesel fuel by way of fuel, it is known practice, in an attempt to combat pollution, to add urea to the exhaust gases, particularly with a view to the reduction of nitrogen oxides.

More specifically, the urea is stored in a tank, in a dilute form diluted in water, and the additive thus formed is injected into the exhaust line of the vehicle, at an appropriate point in this line, that is to say upstream of the catalytic converter in which the reduction of the nitrogen oxides takes place.

It is therefore appropriate to pump the additive from the tank in which it is stored, and to do so in a suitably metered way, and to inject this pumped additive into the exhaust line, at sufficient pressure. The “logical” solution here is to add a metering pump, situated on the additive tank or between this tank and an additive injector positioned on the exhaust line (see, for example, patent documents FR 2 879 239 and WO 00/21881).

Such a solution entails the use of a relatively powerful pump, needed to pressurize a relatively large volume, for example 15 litres, corresponding to the capacity of the additive tank, whereas the flow rate of additive to be delivered remains very low, and needs to be injected only periodically.

Another solution, allowing the pump to be omitted, is to store the additive in a flexible pouch associated with a device that pressurizes the pouch, it being possible for this device to comprise a spring kept under compression and pressed against the said pouch—see document FR 2 870 172 or its equivalent WO 2005/113279, particularly FIG. 2.

However, a system such as this in itself is unable to control either the flow rate or the pressure of the additive and, in particular, does not adapt the pressure of the injected additive to suit the instantaneous requirements. As far as the amount of additive injected is concerned, or put differently, as far as the metering function is concerned, it is necessary to add at the injector an electrically operated valve controlled by a computer.

In practice, the devices for pressurizing and metering additives currently used in association with exhaust systems comprise a tank containing the additive, a (non-metering) pump for pressurizing this additive, and a metering means consisting of an electromagnetic injector which, for each electrical pulse received, delivers a calculated amount of additive.

For metering, the electromagnetic pulse may be of variable frequency and duration. In such a device, the additive pressure supplied by the pump needs to be regulated in order to ensure the accuracy of the volume of additive injected.

The pump, which operates continuously, has therefore to be specified on the basis of its largest flow rate and, when the injector is delivering very little additive, that is to say when the operating point of the combustion engine is at low pollution, the excess additive delivery has to be recirculated, which in itself represents a waste of energy.

In addition, current embodiments of such devices are such that, to ensure that they will operate when the vehicle is starting in extremely cold weather, typically at a temperature of below −11° C., which corresponds to the freezing point of the water-urea mixture, it is necessary to provide a sequence of emptying the additive circuit when the vehicle is stationary and also to locally provide heating means in the additive tank and in the lines.

The current embodiments do not provide an easy solution to the problem of controlling the movements of the additive from the tank to the injector and, during the purge phase, in the opposite direction, from the injector to the tank, irrespective of whether or not the vehicle combustion engine is running. Furthermore, the volume of additive to be drawn back up, which corresponds to the volume of the line leading as far as the injector, inevitably varies according to the vehicle, and a universal device would therefore be desirable.

One solution that has already been envisaged (French Patent Application No. 08.05185 filed on 22 Sep. 2008 in the name of the Applicant) provides, on the additive circuit, a member for pressurizing the additive (the member advantageously consisting of a piston mounted to slide in a cylinder) which is connected by a pipe that taps pressure from the vehicle fuel supply pipe. A pumping device such as this is therefore dependent on the fuel pressure in a line, which pressure is itself associated with the running of the combustion engine, and which disappears as soon as this engine is switched off, which means that a pressure accumulator has to be envisaged.

It is an aim of the present invention to eliminate the various disadvantages set out hereinabove by providing a device for injecting an additive using pumping means suited to the operating conditions and requirements without being dependent on a fuel or any other pressure, these means also being suited to the purge function while at the same time constituting a relatively simple and economical solution to the problem set of pressurizing and metering the additive.

To this end, a subject of the invention is a device for injecting an additive in the liquid state, such as a mixture of water and of urea, into the exhaust system of a motor vehicle with a combustion engine, the additive injection device comprising an additive tank connected by an additive pipe to at least one injector positioned in the exhaust system and a means of pumping the additive being provided on the additive pipe between the additive tank and the injector, these means being capable cyclically of dispensing metered quantities of pressurized additive, the device being essentially characterized in that the pumping means consist of a positive-displacement lift-and-force pump comprising a cylinder, a piston mounted such that it can move translationally in the cylinder and delimiting within this cylinder a pressurizing chamber, a motor or electromechanical actuator coupled to the piston so as to move it translationally, and spring means acting on the piston in the direction of forcing the additive from the cylinder in such a way that actuation of the motor or actuator in one direction causes the piston to retreat, drawing additive into the pressurizing chamber from the additive tank and simultaneously compressing the spring means, whereas the forward movement of the piston delivering the additive to the injector takes place under a controlled pressure, thanks to the said spring means.

Thus, the solution proposed by the invention uses motorized pumping means. Advantageously, the motor of the positive-displacement pump here is a DC electric motor which has the advantage that it can run in both directions of rotation, without difficulty, and that it has a high torque on start-up, this allowing the spring to be compressed easily, and also allowing this spring to be released with ease.

An electric motor such as this is advantageously connected via step-down gearing to a rotary endless screw mounted along the axis of the cylinder of the pump and collaborating with the piston shaped as a nut, or with a nut connected to the piston, the piston and/or the nut being prevented from rotating. A sealing membrane of the “rolling” type may be mounted between the piston and the cylinder of the pump in order fluid tightly to delimit the pressurizing chamber while at the same time being able to cope with the variations in volume of this chamber.

According to an important aspect of the invention, the electromechanical drive means are combined with a spring which, in practice, is a helical compression spring, mounted inside the cylinder of the pump and acting on the piston on the opposite side to the pressurizing chamber. The movement of the piston in the direction for drawing in additive is performed with the aid of the motor or actuator, the power of which needs to be sufficient to compress the spring and at the same time ensure that the pressurizing chamber of the pump is filled, and to do so in a short time, that is to say in a few seconds, for example in about five seconds. The movement of the same piston in the opposite direction, to deliver the additive into the pipe leading to the injector, is performed under controlled pressure thanks to the spring, the latter being rated in such a way as to obtain a small variation in force over the entire stroke of the piston so that the instantaneous flow rate of the injector varies little over this stroke of the piston. The spring in some way acts as a regulator of the pressure and flow rate of the additive during injection.

In a preferred embodiment of the additive injection device according to the invention, there is provided, on the delivery side of the positive-displacement pump, a three-way electrically operated valve, control of which allows the additive to be directed selectively to the injector or, conversely, to the tank in order to empty the additive pipe. The electrically operated valve “manages” the function of drawing up the additive and filling the pressurizing chamber, of delivering the additive, and also of emptying the lines. The device thus has an additional purge function, performed simply, that is to say returning the additive to the tank, thus preventing the lines from becoming blocked with frozen water-urea mixture. For this function of emptying the additive pipe, there is also provided, in combination with the aforementioned electrically operated valve, a venting check valve positioned on this pipe near to the injector.

The cylinder of the positive-displacement pump may comprise heating means, in particular means that work by circulating a hot fluid inside the wall of this cylinder, in line with the pressurizing chamber; these means locally heat the additive, at the pressurizing chamber delimited by the cylinder and the piston of the pump. Advantageously, the aforementioned three-way electrically operated valve is incorporated into the pump or attached to the cylinder of this pump and can thus itself also “benefit” from the aforementioned heating means which defrost the additive.

There may be interposed on the connection between the additive tank and the positive-displacement pump an auxiliary reserve of additive in the form of an annular chamber surrounding the cylinder of the pump in the region provided with the heating means, such that this auxiliary reserve also benefits from the defrosting effect.

Thanks to these arrangements, rapid action of the additive injection device is achieved, even in extreme cold, with priority heating of the water-urea mixture contained in the pump and its closest proximity, it being understood that the heating means call upon a fluid, present in the vehicle, the rise in temperature of which is itself rapid, for example the diesel oil in the fuel return line, or a combustion engine coolant itself heated by exchange of heat. The auxiliary reserve of additive makes it possible to make best use of the heat restored by the heat exchanger formed by the heating means through which a hot fluid thus runs. For preference, this auxiliary reserve of additive has a volume substantially equivalent to the amount of additive injected in an operating cycle of the pump, more particularly a cycle of filling the line that connects the pump to the injector.

In any event, the invention will be better understood with the aid of the description which follows, with reference to the attached schematic diagram which, by way of example, depicts a number of embodiments of this device for injecting an additive into the exhaust system of a motor vehicle:

FIG. 1 is a block diagram of a device for injecting additive according to the present invention, in one position of the operating cycle;

FIG. 2 is a diagram similar to FIG. 1, but illustrating another position in the operating cycle;

FIGS. 3 and 4 are diagrams similar to the foregoing ones and illustrating the operation of the device during purging;

FIG. 5 is a detailed view in longitudinal section of the positive-displacement pump of this device, in one particular embodiment;

FIG. 6 is a sectioned view similar to FIG. 5, but showing another position of the positive-displacement pump;

FIG. 7 is a sectioned view similar to the foregoing ones, showing another embodiment of the positive-displacement pump.

Reference is made first of all to FIGS. 1 to 4 which very schematically show a portion of the exhaust line 2 of a motor vehicle with a combustion engine, particularly a “diesel” engine, into which line an additive with a pollution-reducing function, particularly a mixture of water and of urea, is to be injected.

The additive is stored in an additive tank 3 which is connected, by an additive pipe 4, to an additive injector 5 positioned on the exhaust line 2. Interposed on the additive pipe 4 is a positive-displacement lift-and-force pump 6 designed to draw up the additive stored in the tank 3 and pressurize this additive so that it can be sprayed using the injector 5 into the exhaust line 2.

The positive-displacement pump 6 comprises a pump cylinder 7, in which there is slidably mounted a pump piston 8 which divides the cylinder into two chambers. An electromechanical actuator 9 is coupled to the piston 8 in such a way as to cause it to move translationally, in one direction or the other, inside the cylinder 7.

In one of the chambers delimited by the piston 8 inside the cylinder 7 there is mounted a helical compression spring 10 which bears against one face of the piston 8 so as to push it back to the right (with reference to the drawing). The other chamber 11 delimited by the piston 8 and the cylinder 7 constitutes a chamber for pressurizing the additive. The pressurizing chamber 11 is connected to the additive pipe 4 via a three-way electrically operated valve 12.

Two end-of-travel sensors 13 and 14 are provided on the pump cylinder 7 to detect the arrival of the piston 8 in one or other of its end-of-travel positions.

Finally, a venting check valve 15 is placed on the additive pipe 4, close to the additive injector 5.

FIGS. 1 and 2 illustrate the basic operation of the device, that is to say the cycle that allows additive to be injected into the exhaust line 2. More particularly, FIG. 1 shows the intake phase during which the three-way electrically operated valve 12 is operated in such a way as to place the additive tank 3 in communication with the pump 6, more particularly with the pressurizing chamber 11 of this pump. The electromechanical actuator 9 is then activated to move the piston 8 to the left, the piston 8 thus describing an intake stroke, progressively compressing the spring 10. At the end of its stroke, the pressurizing chamber 11 is filled with additive while the spring 10 is fully compressed. The electromechanical actuator 9 is then switched off.

Next, the three-way electrically operated valve 12 is switched, so as to place the pump 6, more particularly the pressurizing chamber 11, in communication with that portion of the additive pipe 4 that leads to the injector 5—see FIG. 2. The electromechanical actuator 9 is then switched back on, in the opposite direction, so as to release the piston 8 which, now subject to the action of the spring 10, describes a delivery stroke (to the right), thus pressurizing the additive contained in the chamber 11, the pressurized additive being made available in the portion of the additive pipe 4 that leads to the injector 5. The electromechanical actuator 9 is stopped either after a preset operating time or after it has effected a preset number of revolutions, this actuator 9 being particularly an electric motor with two directions of rotation (as specified hereinafter).

Arrival of the pump piston 8 at the end of its delivery stroke, as detected by the sensor 14, triggers the start of a new cycle identical to the preceding one, with a further switching of the electrically operated valve 12 and a further intake of additive into the tank 3, and so on.

On each operating cycle, the device prepares a constant amount of additive, which corresponds to the “swept volume” of the positive-displacement pump 6, in the knowledge that the stroke of the pump piston 8 is constant here. Furthermore, the control system controlling the additive injector 5 makes it possible to determine, by summing the unit quantities injected, the total amount of additive taken from the tank 3 and therefore also, through a simple calculation, the instantaneous position of the piston 8 with respect to its total stroke. The electronic computer (not depicted) which controls the device can then make a correction to the value of the additive pressure, associated with the variation in force of the spring 10 over the stroke of the piston 8.

The purge cycle designed to prevent the risk of blockage in times of extreme cold and, particularly, at temperatures of below −11° C., which corresponds to the temperature at which the water-urea mixture used solidifies, will now be described with reference to the next FIGS. 3 and 4. For this cycle the water-urea mixture is returned to the tank 3, using the same positive-displacement pump 6.

As the vehicle is being switched off, and depending on the detected outside temperature, the three-way electrically operated valve 12 is operated in such a way as first of all to place the chamber 11 of the pump 6 in communication with the tank 3. Thus, the piston 8 already drives towards the tank 3 the quantity of additive that was in the pump 6 at the instant the vehicle was switched off.

Next, when the piston 8 has reached the end of its stroke, the electrically operated valve 12 is switched, thus placing the chamber 11 of the pump 6 in communication with that section of the pipe 4 that leads to the injector 5—see FIG. 3. The actuator 9 is activated in such a way as to move the piston 8 to the left, so that the additive contained in the relevant section of the pipe 4 finds itself drawn into the pump 6, in the knowledge that the check valve 15 then allows air to enter the pipe 4, to replace the removed additive.

The actuator 9 is switched off when the piston 8 has reached the end of its retreat stroke as detected by the sensor 13. The electrically operated valve 12 is then switched and the actuator 9 is switched back on in the opposite direction so as to allow the piston 8 to move to the right, with the aid of the spring 10. The additive previously drawn back in by the pump 6 is then transferred to the tank 3. Emptying is thus performed, by delivering the additive from the pipe 4 to the tank 3.

FIGS. 5 and 6 depict in detail part of the additive injection device, particularly the positive-displacement pump 6 and its electromechanical actuator 9, in one particular embodiment, the elements that correspond to those already described being denoted by the same numerical references.

The actuator 9 here comprises a DC electric motor 16 with two directions of rotation, attached to the pump cylinder 7. The output shaft 17 of the electric motor 16 is connected, via a step-down gearing 18, to an endless screw 19 mounted such that it can rotate about the axis A of the pump cylinder 7. The endless screw 19 collaborates with a nut 20 secured to the pump piston 8, the latter being prevented from rotating relative to the pump cylinder 7. Thus, the rotating of the electric motor 16, in one direction or the other, causes, through the gearing 18 and the screw-nut mechanism 19, 20, the axial translational movement of the piston 8, to the left or to the right, depending on whether the endless screw 19 is being screwed into or unscrewed from the nut 20, so as to deliver or take in additive.

In this embodiment, a flexible sealing membrane 21 of the “rolling” type is mounted between the pump piston 8 and the pump cylinder 7 in order fluid tightly to delimit, at least on one side, the pressurizing chamber 11.

In the region of this pressurizing chamber 11, the wall of the pump cylinder 7 is provided with heating means, in the form of a circuit 22 through which a sufficiently hot fluid that already exists in the vehicle concerned runs, an inlet for this fluid being provided at 23. The fluid in question is, for example, fuel such as diesel fuel returning to the fuel tank of the vehicle concerned. The circuit 22 creates a heat exchanger which allows firstly the additive contained in the pump 6 to be heated in order to obtain a rapid re-entry into service of the device for injecting this additive.

Around the region of the pressurizing chamber 11 there is also provided here an annular chamber 24 which constitutes an auxiliary reserve of additive, receiving at its inlet 25 the additive conveyed from the tank 3 by the pipe 4. This annular chamber 24, and therefore the auxiliary reserve of additive it contains, may benefit from the heating effect resulting from the circulation of a fluid through the circuit 22, because it surrounds this circuit 22.

At the outlet 26 from the annular chamber 24, the additive, possibly heated, is directed towards the three-way electrically operated valve which in this instance is separate from the positive-displacement pump 6.

In an alternative form illustrated by FIG. 7, the three-way electrically operated valve 12 is, on the other hand, incorporated into the positive-displacement pump 6 and, more particularly, positioned at the end of the pump cylinder 7 which is the end adjacent to the pressurizing chamber 11. Thus, the electrically operated valve 12 and the additive it contains may also benefit from the heating effect obtained by the circulation of a fluid through the circuit 22.

The swept volume of the positive-displacement pump 6 will be defined, for each practical application, on the basis of the desired autonomy for the device (expressed in terms of number of kilometres covered by the vehicle) and of the volume of the lines that are to be emptied; for example 30 cm³ corresponding to 30 km covered. The spring 10 of the positive-displacement pump 6 is rated to have a small variation in force over the entire stroke of the piston 8, so that the instantaneous flow rate of the injector 5 varies only little. This flow rate is dependent on the square root of the pressure, which is itself set, for example to a value ranging between 3 and 6 bar, by the spring 10.

Finally, it will be noted that, in the operation of the additive injection device described hereinabove, when the vehicle concerned is stationary, the pump 6 can be immobilized either in a position in which the pressurizing chamber 11 is full of additive, with the piston 8 retreated, or in a position in which the pressurizing chamber is empty, that is to say in which the piston 8 is advanced as far as an end-of-travel position (to the right). The choice of the position of immobilization is here dependent on the strategy adopted for heating and defrosting the additive.

As goes without saying and as is evident from the foregoing, the invention is not restricted only to those embodiments of this additive injection device which have been described hereinabove by way of example; on the contrary, it encompasses all alternative embodiments and applications thereof that follow the same principle.

In particular, the following would not constitute departures from the scope of the invention:

-   -   if, for driving the pump, the DC electric motor were to be         replaced by a motor of any other type, such as an electric         “stepping” motor, or any other electromechanical actuator;     -   if the single three-way electrically operated valve were to be         replaced by any equivalent means, such as two controlled check         valves respectively controlling the inlet and outlet of the         pump;     -   if the detail of the operating sequences were modified;     -   if the additive were heated by circulating any available fluid,         for example engine coolant, or alternatively if use were made of         electric heating means, particularly of the resistive type;     -   if the detailed shapes of the components of the device, for         example those of the pump piston, were modified, and if sealing         around this piston were achieved by any appropriate means, such         as simple annular seals;     -   if the additive injection pressure were adapted to suit each         particular application;     -   if the device were used with any type of additive tank, it being         possible for this tank to be rigid or, on the other hand,         created as a flexible pouch;     -   if the same injection device were to be used for the injection         of any kind of liquid additive, for example with cerine, in         conjunction with any type of combustion engine and fuel, on any         kind of vehicle. 

1. Device for injecting an additive in the liquid state into an exhaust system of a motor vehicle with a combustion engine, the additive injection device comprising: an additive tank connected by an additive pipe to at least one injector positioned in the exhaust system and a means of pumping the additive being provided on the additive pipe between the additive tank and the injector, these means being capable cyclically of dispensing metered quantities of pressurized additive, wherein the pumping means comprise a positive-displacement lift-and-force pump comprising a cylinder, a piston mounted such that the piston can move translationally in the cylinder and delimiting within this cylinder a pressurizing chamber, a motor or electromechanical actuator coupled to the piston so as to move the piston translationally, and spring means acting on the piston in a direction of forcing the additive from the cylinder in such a way that actuation of the motor or actuator in one direction causes the piston to retreat, drawing additive into the pressurizing chamber from the additive tank and simultaneously compressing the spring means, whereas the forward movement of the piston delivering the additive to the injector takes place under a controlled pressure, due to said spring means.
 2. Device for injecting additive according to claim 1, wherein the motor of the positive-displacement pump is a DC electric motor with two directions of rotation.
 3. Device for injecting additive according to claim 2, wherein the electric motor is connected via a step-down gearing to a rotary endless screw mounted along the axis of the cylinder of the pump and collaborating with the piston shaped as a nut, or with a nut connected to the piston, the piston and/or the nut being prevented from rotating.
 4. Device for injecting additive according to claim 1, wherein a sealing membrane of a “rolling” type is mounted between the piston and the cylinder of the pump in order to fluid tightly delimit the pressurizing chamber while at the same time being able to cope with the variations in volume of this chamber.
 5. Device for injecting additive according to claim 1, wherein the spring means acting on the piston in the direction of delivering the additive out of the cylinder comprise a helical compression spring mounted inside the cylinder and acting on the piston on an opposite side to the pressurizing chamber.
 6. Device for injecting additive according to claim 1, further comprising, on a delivery side of the positive-displacement pump, a three-way electrically operated valve, control of which allows the additive to be directed selectively to the injector or, conversely, to the tank in order to empty the additive pipe.
 7. Device for injecting additive according to claim 6, further comprising, for the function of emptying the additive pipe, in combination with the aforementioned electrically operated valve, a venting check valve positioned on the pipe near to the injector.
 8. Device for injecting additive according to claim 1, wherein the cylinder of the positive-displacement pump comprises heating means that work by circulating a hot fluid inside a wall of the cylinder, in line with the pressurizing chamber, these heating means defrosting the additive.
 9. Device for injecting additive according to claim 6, wherein the three-way electrically operated valve is incorporated into the pump or attached to the cylinder of this pump in such a way as to communicate with the aforementioned heating means.
 10. Device for injecting additive according to claim 8, wherein, interposed on a connection between the additive tank and the positive-displacement pump is an auxiliary reserve of additive comprising an annular chamber surrounding the cylinder of the pump region provided with the heating means, such that this auxiliary reserve also benefits from a defrosting effect.
 11. Device for injecting additive according to claim 10, wherein the auxiliary reserve of additive has a volume substantially equivalent to an amount of additive injected in an operating cycle of the pump.
 12. Device for injecting additive according to claim 11, wherein the volume is substantially equivalent to a cycle of filling the line that connects the pump to the injector 